1. Technical Field
The invention refers to a sliding element with a metal supporting layer, with a porous metal carrier layer applied to it and with a sliding layer of thickness grade D applied to the carrier layer, which has at least one thermoplastic material as its matrix material. The invention also refers to a procedure for the manufacture of such sliding elements.
2. Related Art
Maintenance-free bearing materials mostly consist of multiple-layer materials with surfaces made of especially modified plastics. In a frequently used model, the composite materials have a porous metallic component, which is impregnated with a plastic compound, with which it is covered. It can be differentiated here between two types, depending on the matrix plastic: those based on PTFE and those based on thermoplastically processable plastics. The thermoplastic types are either provided with recesses in order to be able to accommodate a permanently acting quantity of lubricant or they are already used in the form of a compound modified with lubricants.
It has become evident that some of these types of materials with their plastic sliding surfaces can also be advantageously used as opposed to purely metal bearing materials in fluid-lubricated applications such as pumps or hydraulic shock absorbers due to their excellent adaptation and dirt absorbing properties and their high resistance to wear. In doing so, the PTFE based materials are characterised by a particularly low coefficient of friction for fluid lubrication, while thermoplastic materials can be highly resistant to wear. However, the thermoplastic materials have a clearly higher coefficient of friction for fluid lubrication, despite their modification.
Thermoplastic bearing materials have been used for many years in particularly wear-intensive fluid-lubricated applications such as described in the DE 32 21 785 C2. The friction bearing composite material is based on PEEK and has been modified with graphite, PTFE and carbon fibres. It is used, for example, in shock absorbers in which permanent stability is more important than a response characteristic with as low a friction value as possible, due to extreme stress.
DE 102 26 264 B4 describes a modified PEEK material with a high resistance to wear, especially developed for high temperatures. The friction bearing composite material has a supporting layer, if necessary a carrier layer and a sliding layer applied to the carrier layer or supporting layer, which has PEEK as the matrix material, a hardening component made of titanium dioxide and/or silicon carbide as well as a lubricant in the form of zinc sulphide and/or barium sulphate. However, the low coefficient of frictions of PTFE based materials for fluid lubrication cannot be achieved with this material.
In DE 198 08 540 84, materials on thermoplastic basis are described, which due to a relatively high PTFE content and the leaving aside of solid additives already have more favorable friction values that are however still clearly above those of PTFE based materials.
The quantity of the PTFE that can be added is limited, as too high a content weakens the thermoplastic matrix, which has a negative effect on the wear resistance of the material.
Two procedures for the manufacture of thermoplastic layers are mainly known:                a) Application of a powder mixture onto the metal substrate with subsequent melting and rolling, as described in the DE 32 21 785 C2.        b) Manufacture of a compound film to roll on to the metal substrate as described in the DE 102 26 364 B4.        
The DE 195 07 045 C2 describes a composite material, for which a paste is applied for the manufacture of the sliding layer having a fluoropolymer e.g. made of PTFE. Other fluoropolymers such as PFA, FEP, ETFE are also provided for, either on their own or in combination.
PTFE layers can also be generated by means of the intermediate step of a film and application to the carrier material or by direct rolling on of an aqueous plastic compound on the basis of a PTFE dispersion, as described in the DE 195 07 042 C2.